CHAPTER XVIII 

 LIPE AS RELATED TO PHYSICAL CONDITIONS 



Ancient Life History. As the rock layers of the earth 

 are explored, fossils of different kinds of plants and animals 

 are discovered. The fossils of the more recent rock layers 



PETRIFIED TREES 

 Found near Holbrook, Arizona. 



correspond very closely to the plants and animals that are 

 found upon the earth to-day, but the older the layers, the 

 less they correspond. There seems to have been a gradual 

 development in life forms through the past ages, a frag- 



522 



